Science and technology

Optical Clocks and Relativity

Do you have the time?

FUNDAMENTAL physics is weird. Some bits of it, say much of quantum theory, are so bizarre that they defy metaphor and only make sense mathematically. Others are more amenable to verbal treatment, which doesn't necessarily make them any less strange. Take Albert Einstein's special theory of relativity. One of its outcomes, which has earned pop-culture notoriety, is the twin paradox. If one twin jets off on a high-speed cosmic jaunt, he will come back younger than his grounded sibling as a result of the difference in their relative velocities. The general theory of relativity, meanwhile, predicts that a clock at a higher altitude will run faster than one below it. That's because according to the theory, gravity is the result of massive objects like stars or planets stretching the fabric of space-time, so the closer a clock is to a massive object, the slower the passage of time.

This effect, called time dilation, can be demonstrated by comparing the tick rates of identical clocks that accompany the different observers. The rub is that at space-time scales humans can get their heads round, the effects are imperceptibly tiny. For instance, in a million years, a clock suspended one kilometre above the Earth's surface would run just 3 seconds faster than an identical one on the surface directly below it.

This hasn't stopped researchers trying. In a famous 1971 experiment Joseph Hafele and Richard Keating took four extremely accurate clocks aboard two commercial airliners that flew around the world twice, eastward and westward, and then compared them to one at the United States Naval Observatory in Washington, DC. On the westward journey, flying against the Earth's rotation, the plane (and the clocks on board) were moving more slowly relative to the Earth's core than the American capital, so time was passing more languorously at the relatively faster-moving Observatory than in the air. The reverse was true on the eastward flight. When flying west, the airborne clocks ran 273 nanoseconds (billionths of a second) ahead of the ground-based one (and so returned to Washington older). In the opposite direction, they ran 59 nanoseconds behind it (in other words, they came back younger). These changes closely matched what Einstein's theory predicted as the cumulative time-dilation effect due to difference in relative speeds and to gravity (as the jets' average cruising altitude was around 8,900 metres).

Now a team of physicists from the National Institutes of Standards and Technology (NIST) in Colorado, led by Chin-wen Chou, has demonstrated time dilation on an even more down-to-earth scale than the Hafele-Keating experiment. According to a report in this week's Science, they used an even more accurate clock to detect a change in tick rates resulting from a difference in speeds of as little as 36 kph (24 mph) and a difference in height of less than a metre.

Obviously not even the finest Swiss timepiece can boast the accuracy required to detect such minute shifts. So physicists have come up with "atomic" clocks that rely on some fundamental physical properties of electrons in an atom. Normally, these reside on specific energy levels but when electromagnetic waves of a particular frequency are shone on them, they absorb energy and jump to a higher level. As they re-emit the energy, they drop back down. By creating a feedback loop which has electrons continuously hopping between two levels, physicists are able to construct an ultrafast and extremely consistent subatomic pendulum. The clocks currently used to set the international time standard rely on microwaves with a frequency of 9.2 billion cycles per second being shone on cesium atoms.

Like older atomic clocks, the NIST device relies on an electron pendulum. However, rather than microwaves, it uses laser light with a frequency of 1,120,000 billion cycles per second shone on a single trapped aluminium ion. This enables it to produce a higher-energy jump, called an optical transition. The upshot is that the new aluminum clock has an accuracy nearly 40 times better than the old cesium sort's three parts in 10 million billion. Einstein's theory may be weird, but what may be weirder still is the fact that people have come up with such mind-bogglingly accurate equipment to prove it right.

Stephen Morris:
Is it an urban myth, or is it really true - as I have read - that the satellites of the GPS system routinely adjust their clocks to account for the effects of General Relativity.

The clocks take into account both Special Relativity (slower than identical Earth based clocks by 7 thousandths of a second per day) and General Relativity (faster than identical Earth based clocks by 45 thousandths of a second per day). The latter is due to the decreasing effect of the Earth on space-time as one move's further away from Earth. The satellite clocks were therefore designed to be slower at launch to compensate. Without this compensation, the accumulated navigational errors would be 10 km per day.

It is worth noting that UT (universal time) is now derived from an atomic reference. This means that it no longer has any fixed relationship to the Earth's rotation and thus to the time of dawn, say (though the practical difference is small). This is because the Earth's rotation speed varies enough to be a nuisance!

All this without the bureaucrats messing about with the clocks twice a year (though come to think of it that might also have been a factor...!)

Einstein's "length contraction" has never been experimentally tested and "time dilation" has only been tested with atomic (i.e. matter based clocks). There are theoretical suggestions that certain testing by beyond non-atomic clocks may show NO time dilation. One lone highly-respected physicist broke from the crowd in mid-20th century and suggested that Einstein special relativity equations could be wrong. All others who said this are considered crackpots who don't understand relativity.But this person no one dares call a crackpot? How come? Details at http://www.physicsnext.org

What I want to say is that Albert Einstein is truly a genius. Relativity is a tough theory, and due to its complex computation, many held the conservative attitude toward it. Nowadays, with the tech. development, this has been corroborated. What a clever man he was. How could this man come up with such idea? He is really amazing. Although time may appear as a stable and solid existence, now we know it is not. The world is so broad that beyond our knowledge, you could hardly tell whether this is a so-called truth or not. Things will change, and so will our conception. Although I have already known this theory, to prove it is another story. On a second thought, what we pursuit is no more than longevity. And it is it. An interesting article.